THE TRAP DYKES OF THE ORKNEYS. 877 



evidence of augite. The ferro-magnesian minerals are much altered, the products being 

 chlorite, calcite, and much anatase. An occasional cluster of epidote and a few grains 

 of sphene are to be found. Magnetite in skeleton networks, ilmenite weathering into 

 leucoxene, and pyrite in large cubes are abundant in some of the slides. 



The order of crystallisation appears to have been as follows : — Pyrite, magnetite, 

 and ilmenite ; apatite ; olivine ; augite ; biotite and hornblende ; and in the groundmass, 

 augite ; hornblende ; plagioclase ; orthoclase. 



The Dyke from Scabra Head, Rousay. — The presence of large phenocrysts, such as 

 occur in the Eennibuster dyke, is not exactly a common feature of the Orkney camp- 

 tonites ; it occurs in all in about a dozen. These are usually 5 feet or more in breadth, 

 but several of the broadest dykes show no porphyritic crystals. On the very summit of 

 Scabra Head, Rousay, there is one which resembles in many respects that of Renni- 

 buster. It is 6 feet broad, and of several dykes in this region it is the broadest, and 

 the only one with porphyritic structure. The edges are fine-grained, but in the central 

 part the dyke is not homogeneous, fine-grained bands alternating with others which are 

 porphyritic. This is perhaps due to enclosures of blocks of sandstone, a few of which 

 are to be seen slightly indurated, and in contact with them the dyke shows a chilled 

 edge. Some of the veins which run in the joints of the flags have a compact edge and 

 a porphyritic centre, though not over 3 inches in total breadth. 



The minerals of the first generation are pyrite in large crystals, magnetite, and 

 apatite ; olivine in small quantity and entirely altered into serpentine ; biotite in scattered 

 scales ; augite and hornblende (PI. I. fig. 4). The augite is perfectly idiomorphic, and 

 this dyke is probably the source of the crystal figured by Heddle, showing the faces 

 (110, 100,010, 111, 10T), as I saw similar crystals in the weathered material on the 

 surface. It encloses olivine, apatite, magnetite, and pyrite. Most of it has a distinctly 

 violet tinge and a slight pleochroism, otherwise it resembles that of Rennibuster. In 

 some of the crystals scattered flakes of hornblende occur, and they have all the same 

 extinction, though few and far apart, being evidently in parallel growth with the augite 

 which surrounds them. There is in many cases a narrow marginal zone of paler colour 

 and slightly different extinction from the central area of the crystal. The hornblende 

 crystals are smaller, but probably more numerous than at Rennibuster, and are more 

 frequently preserved in the sections. Some are almost without cleavage. It is almost 

 quite idiomorphic, in crystals not very elongated. Zonal structure is very common ; 

 mostly the centre is of a darker colour than the margin, but there may be several zones 

 of alternately lighter and darker shade. The various zones have parallel outlines; it 

 encloses apatite, magnetite, pyrite, and sometimes partly envelops an augite. Pleo- 

 chroism and absorption resemble those described for Rennibuster, but the colours are 

 more reddish and the absorption more intense, especially in the interior, where h and t 

 are sometimes dark greenish-brown, a pale brown. 



The groundmass is similar in structure to that of the Rennibuster dyke as regards 

 panidiomorphism and irregular arrangement of the ingredients ; but the white felspathic 



